One of the most common processes for recovery of silver and other precious metals from ore, particularly low grade ores and concentrates is by direct cyanidation. According to this process, a silver-containing ore concentrate, commonly treated with aqueous and pyrometallurgical pretreatment, such as chloride roasting or high temperature aqueous oxidation under pressure with O.sub.2, is leached with a source of cyanide, commonly NaCN. Cyanidation is also applied to some high grade concentrates as an alterative to the accepted smelting procedures. Silver in the ore or concentrate forms the complex Ag(CN).sub.2.sup.- which is soluble in the cyanide leach. After a solid/liquid separation, precious metal values are recovered from the liquid, typically by cementation or carbon extraction.
A number of sources of silver and other precious metals, including gold, are not effectively treated by the direct cyanidation technique. As used herein, "refractory" refers to sources of precious metals in which the precious metals content is associated with MnO.sub.2 or with sulfides or carbonaceous materials and which are thus not amenable to recovery of the precious metals content by direct cyanidation. Refractory sources of precious metals include ores, specifically both raw ores and concentrates, such as flotation concentrates, as well as tailings, both mining tailings and processing tailings such as flotation tailings, and further include cyanide tailings and residuals from various other processes.
Some precious metals-containing ores are refractory because the precious metals are bonded to minerals in the ore. For example, in a manganese dioxide-silver ore, silver is bonded to manganese and direct cyanidation is ineffective to break this bond in any substantial degree. Other types of ores are refractory because precious metals are chemically bound or encapsulated in other minerals. It is not uncommon for even relatively large amounts of precious metals to be found in a sulfidic ore in a highly dispersed form so that there is little surface area of precious metals exposed to a leach. Recovery of precious metals from such ores by, e.g., cyanidation often involves chemical or pyrometallurgical alteration or performing particularly fine grinding on the ore. Such ores include pyrite- or chalcopyrite- antimony- or arsenic-containing ores. Yet another group of ores is refractory because the gangue material, far from being resistant to reaction with cyanide, is particularly reactive with cyanide so that in order to obtain satisfactory dissolution of precious metals, very large amounts of cyanide reagents must be added. A number of methods for treating manganese dioxide-silver refractory ores have been proposed, but most of these are not presently economically viable and few have progressed beyond an experimental stage. Various methods for treating sulfidic and carbonaceous ores are practiced or have been proposed, but for many of these ores, the economics of such methods are unsatisfactory.
Among the methods attempted for treatment of manganese dioxide-precious metals ores are a number of two-stage processes involving a variety of pretreatment methods followed by cyanidation. The pretreatment methods are intended to weaken or break the manganese-precious metals bonds in order to render the precious metals amenable to cyanide dissolution. These pretreatment methods include SO.sub.2 leaching, roasting reduction and salt roasting. These and other methods such as copper sulfate plus sodium chloride treatment followed by mercury amalgamation, thiosulfate leach, direct smelter recovery, chloridizing reduction, segregation roasting, flotation, and brine leach have been reviewed in C. D. Chase, "Treatment of Manganiferous Silver Ores for Recovery of Silver in View of Changed Precious Metal Economics", in Gold and Silver-Leaching, Recovery and Economics, W. Schlitt, W. Larsen, J. Huskey, Chapter 3, pp. 23-33.
Another class of methods for treatment of refractory manganese dioxide and other types of precious metals ores depends on a brine leach step. It is known that silver, in its native state, or at least after it has been dissociated from such refractory materials as sulfides and manganese oxides, will, in the presence of excess chloride ions, such as are present in a brine solution having a chloride concentration of 5-6 molar, form a tetrachloro complex (AgCl.sub.4.sup.-3), soluble in a brine solution. Gold, under conditions of high oxidizing potential, similarly forms soluble auric chloride (AuCl.sub.4.sup.-). One method of assuring a Cl.sup.- concentration which is sufficiently high to dissolve precious metals is to add a reagent such as NaCl, CaCl.sub.2, or HCl. P. R. Bremmner, "Silver Recovery from Cyanide Tailing Using an Acidic NaCl-FeCl.sub.2 Leachant", Bureau of Mines Report No. 8649 describes a leaching process performed on the tailings from a cyanide process. The tailings contained 1.7 oz./ton silver and 0.025 oz./ton gold. Manganese content was approximately 0.4 percent. A silver extraction of 47 percent was obtained by a 24-hour leach in 5 weight percent NaCl and 1.2 weight percent hydrochloric acid. Silver extraction increased to 82 percent with the addition of 2.5 weight percent FeCl.sub.2 as reductant to dissociate the manganese and silver. Acid consumption was 40 to 46 lbs. HCl per ton of feed.
Many brine leach methods are conducted in an acidic environment, particularly in the presence of HCl. One difficulty with a hydrochloric acid leach process, particularly when precious metals content is low, such as less than 5 oz./ton, is the expense of providing the HCl consumed in dissociating the ore. In processes which are directed to recovery of manganese, rather than recovery of precious metals, it has been suggested that HCl used in a manganese ore leach process be partially provided by regeneration of HCl through pyrohydrolysis of the MnCl.sub.2 product. L. D. Normal and R. C. Kirby, "Review of Major Proposed Processes for Recovering Manganese from United States Resources, Part II", Bureau of Mines Information Circular 8160; U.S. Pat. No. 4,284,618, issued to Van der Heyden et al. on Aug. 18, 1981. Pyrohydrolysis of MnCl.sub.2 to HCl has also been suggested in connection with steel pickling and other mineral and metallurgical processing. A. Conners, "Hydrochloric Acid Regeneration as Applied to the Steel and Mineral Processing Industries", CIM Bulletin, February 1975, pp. 75-81. Such pyrohydrolysis is not practical in conventional brine leach methods of precious metals recovery, because those methods require addition of reagents such as NaCl to dissolve precious metals values. The resultant pregnant liquor contains significant concentrations of NaCl, which, upon heating, will not dissociate chloride. Such a process would thus involve high chloride and HCl consumption.
One alternative to the use of relatively expensive HCl is a H.sub.2 SO.sub.4 -NaCl leach. Previous tests of such a system have indicated silver extractions from manganese dioxide ores were around 51 percent, compared to an 84 percent extraction obtained from an SO.sub.2 -NaCl treatment in which SO.sub.2 acts as a reductant and provides the sulfuric acid environment. B. J. Scheiner et al., "Extraction of Silver from Refractory Ores", Bureau of Mines Report of Investigations 7736. A sulfuric acid leach of manganese ores which is directed to extraction of manganese, rather than recovery of precious metals, has been found to improve upon addition of pyrite or lignite char. V. G. Leak, "Autoclave and Ambient Pressure Leaching of Lake Superior of Manganiferous Ores", Bureau of Mines Report of Investigations 7501.
The acid processes for recovering precious metals from manganese dioxide precious metals ores normally require a process for neutralizing and rejecting manganese and this is another expense.
Ores which are refractory because of the association of precious metal values with sulfidic or carbonaceous materials may also be treated by a two-step process involving a pretreatment followed by cyanidation. Pretreatment methods which have been attempted include oxidation roasting and wet oxidation at elevated temperatures under an O.sub.2 overpressure or with Cl.sub.2. B. J. Scheiner et al., "Oxidation Process for Improving Gold Recovery from Carbon-Bearing Gold Ores", Bureau of Mines Report of Investigations 7573 discuss recovery of gold from a carbonaceous ore, including oxidation with chlorine, sodium hypochlorite or calcium hypochlorite, followed by cyanidation. As with the two-step treatments directed to the manganese dioxide-precious metals ores, two-step processes are, in general, more expensive to implement and operate than a one-step process. Additionally, oxidation roasting typically requires expensive and technically demanding methods for controlling the release of SO.sub.2 and other toxic compounds such as arsenic into the environment, while wet oxidation with O.sub.2 overpressure requires high capital and operating costs.
After pretreatment, sulfidic or carbonaceous precious metals ores have also been treated by a brine leaching process. Such a process typically requires high chloride concentrations to maintain the chloride solubility of precious metals which are often relatively high in concentration in sulfidic/carbonaceous ores or concentrates. This leads to costly loss of chloride due to washing requirements. Gold solubility in chloride solution also requires addition of a powerful oxidizing agent which adds to the cost.
In summary, previous methods have not effectively dealt with a number of problems associated with refractory precious metals ores. One difficulty with previous method of precious metals recovery from refractory ores has been the high cost of reagents, compared to the value of recoverable precious metals. This problem is particularly acute in ores which contain a modest amount of silver, for instance less than about 10 oz. Ag/ton of ore. When precious metal values are associated with MnO.sub.2, reagent costs of providing a reductant for reducing the manganese in the ore can be prohibitive. Similarly, when precious metals are associated with sulfides in the ores, an oxidant for the sulfides must be provided, at a cost which may be prohibitive.
Reagents which are customarily provided to maintain the chloride ion concentration sufficiently high to render the precious metals soluble in the leach liquor represent another cost which, in many cases, can be excessive. Acids such as H.sub.2 SO.sub.4 and, particularly, HCl represent a high reagent cost for previous precious metals recovery methods.
Methods which require a cyanide treatment also require costly methods for controlling the release of cyanide compounds into the environment, as well as the cost burden of a two-step process.
Previous methods for recovery of precious metals from refractory ores typically render recovery of by-products such as zinc, lead or copper difficult or infeasible.
Accordingly, it is an object of this invention to provide a process for recovering precious metals from a refractory ore which requires minimal cost or consumption of reagents, such as acids, oxidants or reductants and chloride sources and minimal capital and operating costs.
It is another object of this invention to provide for a precious metals recovery process which can produce marketable base metal by-products in addition to precious metals.
It is a further object of this invention to provide for a refractory ore, precious metals recovery process in which chloride values are recycled in the process.
It is a still further object of this invention to provide for a precious metals recovery process which minimizes the expense associated with environmental controls.
It is yet another object of this invention to provide a precious metals recovery process in which MnO.sub.2 is reduced in an acid leach and a sulfidic or carbonaceous material is oxidized.